Structural
engineer describes what went wrong inside the World
Trade Center on Sept. 11

Vulnerabilities
in the design of New York's World Trade Center (WTC)
are likely to have contributed to the collapse of its
two main towers and adjacent buildings, according to
Ronald O. Hamburger, a structural engineer currently
investigating the Sept. 11 disaster.

"These
buildings were incredibly strong, especially with
respect to resisting dead loads and wind loads, but
they also had a number of vulnerabilities,"
Hamburger told a packed auditorium on Nov. 29 when he
delivered the second John A. Blume Distinguished
Lecture ­ an annual event sponsored by Stanford's
Blume Earthquake Engineering Center.

"What
New York City experienced on Sept. 11 was very much
like an earthquake," he told the Stanford
audience. "Life loss exceeded anything we in the
United States have experienced in an earthquake, and
the financial loss exceeded anything we've
experienced ­ and it all occurred within one square
mile."

As chief
structural engineer and senior vice president of ABS
Consulting Inc. in Oakland, Calif., Hamburger is a
member of an engineering team commissioned by the
Structural Engineers Institute of the American
Society of Civil Engineers (ASCE) to assess the
performance of the WTC and surrounding buildings in
the aftermath of the terrorist attacks.

He
pointed out that four buildings were immediately
destroyed in the WTC assault, and three others
suffered irreparable damage and are in the process of
being razed. Another half-dozen buildings were harmed
structurally but can be repaired, and more than 50
others were damaged by the enormous debris cloud and
the burning material that followed the collapse of
the twin towers.

"Thirty
million square feet of commercial space were
affected, including 10 million square feet that were
taken out permanently ­ the equivalent of all the
financial office space in the city of San
Francisco," he noted.

Hijackers'
plan

Using
photographs of the WTC taken before, during and after
the Sept. 11 assaults, Hamburger presented a brief
chronology of events. He noted that the first
jetliner banked into the north tower at a 45-degree
angle, damaging floors 92 to 95. About 40 minutes
later, the second jet crashed into the south tower,
hitting floors 78 to 84.

"I
believe that the hijackers flew the aircraft into the
lowest part of the buildings they had access
to," Hamburger commented. "If there had
been no nearby structures, they would have hit the
towers lower."

One
member of the audience asked Hamburger if he believed
that the pilots intentionally banked the planes at an
angle to take out as many floors as possible.

"Yes,"
he replied.

According
to Hamburger's preliminary analysis, the impact of
the jetliners shattered and fractured two-thirds of
the support columns on one face of each tower,
causing the partial collapse of several floors.
Debris penetrated each building's core and may have
damaged the core columns located in the center of the
110-story structures.

"The
damaged columns held up the weight of the building,
so logic would dictate that the building would
fall," said Hamburger, "but that didn't
happen. Because of its great structural redundancy,
the load was distributed to other parts of the
building. We have reason to believe that, without the
fire, the buildings could have stood indefinitely and
been repaired. But we did have a fire."

Born
of fire

Hamburger
noted that the fuel in both jetliners burned off
rapidly, despite media reports that the aircraft
continued burning long after the crash.

"The
impact probably caused a failure of the fireproofing
in the affected areas," he said. "We think
that the fuel ignited several floors in the
building," he added, which had a devastating
effect on the steel support beams.

"Steel
is born of fire," Hamburger explained. "As
it's reheated, it expands and loses its rigidity.
Above 1,000 degrees Fahrenheit, it loses a
significant amount of its strength."

He said
the extreme heat from the fires might have caused the
steel floors to expand and bow, which may have caused
the support columns to bend inward and buckle. Heat
also may have caused the steel flooring to separate
from the columns, or the columns themselves may have
heated up and buckled outward.

Hamburger
and his colleagues have not yet determined which of
these scenarios occurred on Sept. 11, but there is
little doubt that the collapse of the upper floors of
the WTC towers brought down both structures.

"Think
of the impact of dropping a 25-story building
straight down," Hamburger told the audience.
"It was like a pile driver, which is why it
collapsed as it did."

Vulnerabilities

While
acknowledging the many innovations that went into the
design of the WTC towers in the 1960s ­ including
one of the earliest applications of computer stress
analysis ­ Hamburger also cited several features
that made the buildings vulnerable to the intense
fires that ultimately caused their collapse.

"The
floor trusses [joists] were relatively flimsy. As the
tower collapsed, the trusses just fell apart,"
he observed, noting that trusses are difficult to
fireproof.

Hamburger
noted that each tower was constructed using a novel
tube frame system designed to resist winds of up to
80 miles per hour. But the connections of the tube
frame were weak, causing them to break apart and
become three-pronged missiles that crashed into the
street and into nearby buildings.

Hamburger
also discussed the collapse of WTC Building 7, which
housed the offices of Con Edison, the FBI and the
CIA.

"WTC-7
was a 47-story building and became a two-story pile
of rubble," Hamburger said, "making it the
first major structure in the United States to
collapse because of fire."

Future
designs

He
pointed out that fires frequently occur in high-rise
buildings and noted that between 1994 and 1998, 30
fires occurred in the United States in buildings that
were 50 stories or taller.

"The
question is, should fire protection standards be
changed in some significant way in the aftermath of
Sept. 11," Hamburger asked, "and should
structural engineering designs include consideration
of fire load and the response of structures? Right
now, structural engineers know very little about
fire."

He and
his colleagues will attempt to answer those questions
when the ASCE report is made public next spring.

Does it
make sense to design buildings to withstand such
events as Sept. 11 in the future?

"In
my opinion, no," Hamburger concluded, although
he does support the decision implemented after the
1995 Oklahoma City bombing in 1995 to redesign
federal buildings to withstand the accidental loss of
major structural elements.

The
Blume Lecture series honors John A. Blume, often
called "the father of earthquake
engineering," who earned his undergraduate and
doctoral degrees at Stanford. In 1974 he helped
underwrite the John A. Blume Earthquake Engineering
Center, which promotes earthquake engineering
research and education. Today at age 92, Blume
remains a consulting professor in Stanford's
Department of Civil and Environmental Engineering.